Principles and Practice of
`RADIATION
`
`ONCOLOGY
`
`Third Edition
`
`Edited by
`
`Carlos A. Perez, M.D.
`Director, Radiation Oncology Center
`Mallinckrodt Institute of Radiology
`
`Washington University Medical Center
`St. Louis, Missouri
`
`Luther W. Brady, M.D.
`Hylda Cohn/American Cancer Society Professor of Clinical Oncology
`and Professor, Department of Radiation Oncology
`Allegheny University Hospitals-Hahnemann
`Philadelphia, Pennsylvania
`
`103 Additional Contributors
`
`Assistants to the Editors
`Connie Povilat
`Alice Becker
`
`R Lippincott - Raven
`
`Philadelphia - New York
`
`
`
`Page 1 of 27
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`
`3rd Edition
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`Library of Congress (Iatalogitig-in-PuhlicaLion Data
`
`Principles and practice of Imitation oncology/[edited by] Carlos A.
`Perez, Luther W. Brady; with I03 additional contributors;
`iissistatits to the editors. Cotinie Povilzit. Alice l3ceker.—-3rd ed.
`p.
`cm.
`includes bibliogruphictil references and index.
`ISBN 0-397—58416~4
`1. (‘iiricer—Radtothcrapy.
`ll. Brady. Luther W.,
`l025—
`[DNLM:
`1. Neoplasins—tadiothctripy. Q7. 26‘) P957
`RC27l.R3P73
`I997
`6l6.9‘)’40642 —dc2l
`DNI .M/'[)LC
`for Library of Congress
`
`I. Perez. Carlos A..
`
`l934—
`
`1997]
`
`97-3105
`CII’
`
`Cure lit-is heeii tzilten to confirm the accuracy of the intormatum presented and to describe gcncrnlly
`accepted pritcttc:-s.
`iluwever.
`the authors. editor.~., and publisher me not responsible for Clt’t)I’S tll
`omissions or for an) coiiscqttenccs trout zippliczttion of the iii1'otiii:iiion in this book and nizike no
`warranty. express or tiiiplictl. with respect in the L‘0l1lL’nl~' of the ptihliczttton.
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`l'oi‘th in this text are in accordance with current |'CCUltlil]t:l]L.l.llit)llS and pmctici: zit
`the
`tinic of publication. However. in View of oiigoitig l'L‘\"t'tll(.‘i1. clizinges in govcmment regulatioiis. and
`the constant flow of iitfuriiiiitiiiii teltitiiig to drug therapy and drug reactions. the reader is urged to
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`Some thugs illlll inetlieail devicex‘ presented iti this publication have Food iitid Drug Adtiiinistixttioti
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`Page 2 of 27
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`

`
`l’r'm<‘ipIt-.< mnl ,’Ilf('fI1'I’ u_I’RmIi'ulion ()Itcui’t>gv_
`l'lItr.I Izillrmn cdiletl by (' A. Pen‘? and l.. W. Hrzttly.
`l.tppint'otI~-Ra\'L'ii Pitlvlixlicrs. Pltil;tt‘elp|-.ia 1* IW7.
`
`CHAPTER 1
`
`Overview
`
`Carlos A. Perez, l.t1ther W. Brady, a11(l_]0Stfpl1 L. Roti Roti
`
`
`
`HISTORICAL PERSPECTIVE
`
`The year I995 marked the centennial of Roentgcn‘s dis-
`covery of x-rays in I895.” The Curies rcponcd their
`discovery of radiutn in 1898.“ Almost immediately. the
`biologic effects of ionizing radiations were recognized;
`the first patient cured by radiation therapy was reported
`in 1899, after which clinical radiation therapy had a chal-
`lenging growth period in the early 1920s. Clinical and
`technologic advances accumulated tnore rapidly thatt did
`basic biologic knowledge.
`Clinical radiation therapy as a medical discipline began
`the lntemational Congress of Oncology in Paris in
`at
`1922 when Coutard and Hautant presented evidence that
`advanced laryngeal cancer could be cured without disas-
`trous, treatment-induced seqtielaef“ By l934, Coutard‘""’
`had developed a protracted, fractionated scheme that re-
`mains the basis for current radiation therapy, and in l936
`Paterson"'” published results in the treatment of cancer
`with x—rays. The use olibrachytherapy, starting with ”"Ra
`needles and tubes. has increased steadily since l9l0 in
`the treatment of malignant tumors in many anatomic loca-
`tions. With time. ionizing radiation became more precise.
`high-energy photons and electrons were available. and
`treatment planning and delivery became more accurate
`and reproducible.
`radiation biology,
`Knowledge of radiation physics,
`clinical treatment planning. and the use of computers in
`radiation therapy has grown exponentially. The last two
`decades have witnessed considerable advances in the
`treatment ofcancer, with cure now being a realistic thera-
`peutic objective in over 50% of newly diagnosed pa-
`tients.‘”'“"’ This improvement in therapy can be attributed
`to progress in several major areas:
`
`1. Greater dissemination of information to physicians
`and the public and innovative screening and diagnos-
`tic tools that
`increase awareness and early cancer
`detection.
`
`[Q . Multiple therapeutic. approaches
`tumors.
`
`for a variety of
`
`3. Advanced surgical and irradiation techniques and more
`effective cytotoxic drugs.
`radia-
`4. Greater
`interaction among cancer surgeons.
`tion oncologists. medical oncologists, and patholo-
`gists. stressing the cotnbined-modality approach in
`treatment.
`
`5. Closer interaction among physicians and basic scien-
`tists, allowing the transfer of clinically relevant bio-
`medical discoveries to the bedside.
`6. Broad use of clinical trial methodology to evaluate
`new therapeutic strategies.
`
`RADIATION ONCOLOGY IN CANCER
`MANAGEMENT
`
`Radiation mtculugy is a clinical and scientific discipline
`devoted to management of patients with cancer and other
`diseases by ionizing radiation, alone or combined with
`other modalities. investigation of the biologic and physi-
`cal basis of radiation therapy, and training of profession-
`als in the field. Radiation therapy is a clinical modality
`dealing with the use of ionizing radiations in the treatment
`of patients with malignant neoplasias (aitd occasionally
`benign diseases). The aim ofradiation lllL‘l‘:Ip_V is to deliver
`a precisely measured dose of irradiation to a defined tu-
`mor volume with as minimal damage as possible to sur-
`rounding healthy tissue, resulting in eradication of the
`tumor. a high quality of life. and prolongation of survival
`at competitive cost.
`In addition to curative efforts. radiation therapy plays
`a major role in cancer management
`in the effective
`palliation or prevention of symptoms of the (liseasc:
`pain can be alleviated. luminal patency restored. skele-
`tal integrity preserved. and organ function reestahlislteil
`with minimal morbidity iii
`a variety of clinical
`circumstances.’“’
`
`
`
`Page 3 of 27
`
`

`
`2 ,/ PR|.\'(IIPl.l-ZS AND PRA(1‘l‘l(Il-‘. or R,\t>i.»\'rioN ()Nc:t:>t.tx;v
`
`In 1962. Busclilte“ defined a radiotherapist as a physi-
`cizin whose practice is litnited to radiation therapy: he
`emphasized the active role of the radiation oncologist:
`
`is under our care we take full and
`While the patient
`exclusive respotisibility, exactly as does the surgeon who
`takes care of at patient with cancer. This means that we
`examine the patient personally. review the microscopic
`matei'ial. pertomi exainiiintioiis and tailte ti biopsy il‘ nec-
`essary. On the basis of this thorough clinical iitvestigtition
`we consider the plan oi" trcatiiieiit and siiggest
`it to the
`i-el'ertiiig physician and to the patient. We reserve for
`ourselves the right to an iiiilepeiitleiit opinion regarding
`tliiigiiosis and advisable therapy and il'tiecesszii'y. the right
`ofdisagrceinciit with the i'el'et‘iiiig ])lly.\lL‘.l:ln.
`.
`.
`. During
`the course of treatment. we ourselves direct iuiy iidditioiizil
`medication that may he iieeessatry .
`.
`. and are ready to
`he Called in an e,m,rg¢,,Cy at any t;m¢_
`Buschke went on to indicate that. in ortler to integrate
`the various disciplines and provide better care to patients,
`it is extremely important for the radiation therapist (now
`
`oncologist) to cooperate closely with specialists in other
`fields in the management of the patient.
`These concepts were reinforced and amplified by
`Bush” in his dissertatinii on “The Compleat Oncologist"
`atid del Regato” in his I975 ASTR presidential address.
`Today mdimion Oncology is recognized as 3 separate
`Specially by [he American Board of Radiology‘ the Amcr_
`‘Call C°'_]¢t_1_¢ "ti R?1d10’0g_)’- thc Amfiflcéln Board of Medi-
`CF11 S|7€C|‘<|lUC-*3 [he AmCl'|C3“ C0“°2:’C "fR3d13l10n O"‘°1'
`ogy, and the American Medical Assoeiatioii.
`
`‘
`‘
`‘
`I
`‘
`‘
`1 HE PROCESS OF RADIAUON 1 "ER-APY
`
`_
`_
`_
`_
`_
`.
`The clinical use of irradiation is a eomplexprocess that
`involves many professiotials and a variety 0! interrelated
`fiiiielions (Fig.
`l—l). The aitn of therapy should be de-
`lined at the onset of the formulation of therapeutic strat-
`egy as follows:
`
`KEY STAFF
`
`SUPPORTIVE ROLE
`
`1. CLINICAL EVALUATION
`2. THERAPEUTlC DECISION
`3. TARGET VOLUME LOCAUUTION
`
`Flad. Oncologist
`Rad. Oncologist
`
`Ttirnor Volume
`Sensitive Critical Organs
`Patient Contour
`4. TREATMENT PUANNING
`
`Beam Data-Computerization
`Computation of Beams
`Shielding Blocls,
`Treatment Aids. etc.
`Analysis oi Alternate
`Plans
`Selection of Treatment
`Plan
`Dose Calculation
`5. SIMULATIONNERIFICATION
`OF TREATMENT PLAN
`6. TREATMENT
`
`First Day Set-Up
`
`Localization Films
`
`Dosimetry Checks!
`Initial Chart Review
`I-‘iepositiontng’Fietreatm-ant
`
`7. PEEIODIC EVALUATION
`(DUW19 Treatment)
`Tumor F.esponse1Tolerance
`a. FOLLOW-UP E-/ALUATEON
`
`Rad. Oncologist
`Rad. Oncologist
`Dosimetrist
`
`Sim. Tech./Dosimetrist
`Sim. Tech./Dosimetrist
`Sim. Tech./Doslmetrist
`
`Physicist
`Physicist
`Dosimetristl
`Mold Room Tech.
`Rad. Oncologistl
`Physicist
`Rad. Oncologist!
`Physicist
`Dosimetrist
`Rad. Oncologist!
`Sim. Tech.
`
`Rad. Oncologistl
`Physicist!
`Therapy Techs.
`
`Fiad. Oncologist!
`Therapy Techs.
`Physicist]
`Rad. Oncologist
`Therapy Techs.
`
`Dosimetrist
`Ftad. Oncologist!
`Physicist
`Dosimetrist
`
`Physicist
`Dosimetristl
`Physicist
`
`Dosimetristi
`Physicist
`
`Dosimetristl
`Chief Tech,
`Dosimetristl
`Chief Tech.
`
`F-‘tad. Oncologist
`Ftad. Oncologist
`
`Nurse&'.‘Tl'Ts
`Nurses
`
`Page 4 of 27
`
`FIGURE 1-1. Functions involved in radia-
`tion therapy. (Inter-Society Council for Radi-
`afio” On°°'°gy: Radiation 0”°°'°9V in mt?‘
`grated Cancer Management’ Ph"ade'ph'a'
`PA, American College of Radiology. 1986)
`
`

`
`]_
`
`is.)
`
`('urutiv('_ in which it is projected that the patient has
`2| probability of long-tenn survival after adequate ther-
`apy, even it” that chance is low (L15 in T4 tumors of the
`head and neck or carcinoma of the lung).
`l’crlIiatirw,
`in which there is no hope of the patient
`surviving for extended periods‘. n6\"Cl‘lhc‘/ICSS symp-
`toms that produce discomfort or an impending condi-
`tion that may impair the comfort or self-strfficiency oi’
`the patient require treatment.
`
`in curative therapy a certain probability of significant
`side effects of therapy. even though undesirable, may be
`acceptable. However.
`the same is not generally true in
`palliative treatment. in which no major iatrogenic condi-
`tions should be seen. Nevertheless, it is riecessary to re-
`member that in the palliation of primary tumors. relatively
`high doses of irradiation (sometimes 75% to 80% of cura-
`tive dosc) are required to control the tutnor lor the survival
`period of the patient.
`is extremely important for the
`in a curative setting it
`radiation oncologist to deliver the highest possible dose
`to the ttrrnor volume to ensure maximum tumor control,
`while keeping at
`the lowest possible level any severe
`sequelae of radiation treatment in the. surrounding normal
`tissues. The prescription of irradiation is based on the
`following principles:
`
`P“
`
`I. Evaluation of the full extent of the tumor (staging)
`by whatever means available, including radiogrrrphic,
`radioisotope, and other studies.
`2. Knowledge ol‘ the patlrologie cliztractcristics of the dis-
`ease. including potential areas 01' spread. that may in-
`llucnce choice of therapy (ie, rationale for elective
`irradiation of the lymphatics in the neck or pelvis).
`Dclinition of goal of therapy (cure versus palliation).
`4. Selection of appropriate treatment modalities, which
`may be irradiation alone or irradiation combined with
`surgery. chemotherapy, or both. The choice will have
`a significant
`impact on the volume treated and the
`doses of irradiation delivered.
`. Determination of the optimal dose of irradiation and
`the volume to be treated, which are made according to
`the anatomic location. histologic type, stage. potential
`regional nodal involvement. and other cliaracteristics
`of the tumor, and the normal structures present in the
`region. The radiation oncologist should never hesitate
`to modify established policies in order to tailor the
`treatment plan to the needs of the patient.
`6. Evaluation of the paticnt‘s general condition. periodic
`assessment of tolerance to treatment. tumor response.
`and status of the normal tissues treated.
`
`’Jt
`
`The radiation oncologist must work closely with the
`physics, treatment planning. and dosimetry stalls to en-
`sure the greatest possible accuracy, practicality, and cost
`bcnelit in the design of treatment plans and computation
`of dose distributions. The ultimate responsibility for treat-
`
`(IllAl"ll£R 1: O\'l-‘.RV''ll-;\~\’
`
`_/ 3
`
`meat decisions and the technical execution of the therapy,
`as well as its consequences. will always rest with the
`radiation oncologist. W No computer calculations or phys-
`ics procedures will cot'rect the errors ofclinical judgment,
`misunderstanding of physical concepts, or unsatisfactory
`planning and execution of radiation therapy. The skills
`of the clinician will never be completely replaced by tech-
`nologic developments in physics. computers, or other
`technical aspects of Hlkiiillitlll therapy: however. more are-
`curate technrques and quality assrrrauce procedures will
`ensure that the best possible treauucnt is being executed
`and that
`the possibility of subjective interpretations or
`inaccuracies is reduced to a mitrirntrm.
`
`IRRADIATION TRlCATMEN'l' PLANNING
`
`It should be stressed that different doses of radiation
`are rcqtrired for given probabilities of tumor control, de-
`pending on the type and initial number of clonogcnic
`cells present. 'l"herel'orc. varying radiation doses may be
`delivered to certain portions of the tumor (periphery ver-
`sus central portion) or in cases in which all gross tumor
`has been surgically rcmovcd.‘“
`From a cell burden standpoint. a clinical turrror can be
`considered to encompass several compartments: macro-
`scopic (visible or palpable). microextensions into adjacent
`tissues. and subclinical disease, presumed to be present
`btrt not detectable even under the microscope. Treatment
`portals must adequately cover all three compartments in
`addition to a margin to compensate for geometric inaccu-
`racies dtrring irradiation exposure.
`According to International Commission on Radiation
`Units and Measurements (ICRU) No. 50,332 volumes of
`interest
`in treatment planning are defined as follows.
`Gross tumor volume (GTV) is all known gross disease
`including abnormally enlarged regional
`lymph nodes.
`When GTV is determined, it is important to use the appro-
`priate computed tomography (CT) window and level set-
`tings that give the rnaxirnurn dimension of what is consid-
`ered potential gross disease. Tire clinical target volume
`(CTV) encompasses the GTV plus regions considered to
`harbor potential microscopic disease. The planning target
`volume (PTV) provides a margin around the CTV to
`allow for variation in treatment setup and other anatomic
`motion during treatment such as respiration. The PTV
`does not account for treatment machine beam characteris-
`tics (Fig. 1—‘2)."“""3
`Sensitive structures within the irradiated volume should
`be clearly identified. and the maximum doses and frac-
`tionation to be delivered to them must be specified. Simu-
`Iatiou has been used in most instances to accurately iden-
`my the tumor volume and sensitive structures and to
`document the configuration of the portals and target vol-
`ume to be irradiatcd.'”
`Perez and associates“ described the conceptual struc-
`
`
`
`Page 5 of 27
`
`

`
`4 / PRtNctPt.t:s AND PRACTICF. or RADlA‘l‘l()N ONC()l.()(}Y
`
`DEFINITION OF "VOLUMl:'S“
`IN RADIATION THERAPY
`
`TUMOR/TARGET VOLUME
`
`C) Planning
`
`A) Cross
`B] Clinical
`
`D) Treatment portal
`
`'IAR(Il;'l VOLUMES
`
`FIGURE 1-2. Schematic representation of “vol-
`umes" in radiation therapy. The treatment portal
`volume includes the tumor volume. potential areas
`or local and regional microscopic disease around
`the tumor. and a margin of surrounding normal tis-
`sue. (Moditied from Perez CA. Purdy JA: Rationale
`for treatment planning in radiation therapy. In Levitt
`SH, Khan FM, Potish HA. ed: Levitt and Tapley's
`Technological Basis ot Radiation Therapy: Practi-
`cal Clinieal Applications, ed 2. Philadelphia, PA,
`Lea & Febiger, 1992)
`
`ture and process of a fully integrated three-dimensional
`(3-D) CT simulator. The elements of an optimal device
`include (a) volumetric definition of tumor volume and
`patient anatomy obtained with a CT scanner, (b) virtual
`simulation for beam setup and digitally reconstructed ra-
`diograplts. (c) 3-D treatment planning for volumetric dose
`computation and plan evaluation. (d) patient-marking tie-
`vice to outline portal on patient's skin. and (e) verification
`(physical) simulation to verify portal placement on the
`patient. Average time for CT volumetric simulation was
`74 minutes without or 84 minutes with contrast material.
`Average times were 36 minutes for contouring of tumor/
`target volume and 44 minutes for normal anatomy. 78
`minutes for treatment planning, 53 minutes for plan evalu-
`ation/optimization, and 58 minutes for verification Simu-
`lation. There were significant variations in time and effort
`according to the specific anatomic location of the tumor.
`Commercially available CT simulators lack some ele-
`ments that were believcd to be critical in a fully integrated
`3-D CT simulator. Further efforts are in progress to de-
`velop more versatile and efficient 3-D simulators. Based
`on actual budgetary information, the cost of a volumetric
`CT simulation (separate from the 3-D treatment planning)
`showed that l2()0 examinations per year (four to five per
`day in 250 working days) ideally should be performed to
`make the operation of the device cost effective.
`Treatment aids. such as shielding blocl-ts. molds. masks.
`immobilization devices. and compensators. are extremely
`important in treatment planning and delivery of optimal
`dose distribution. The radiation oncologist should be fa-
`miliar with the physical characteristics of these devices
`and use them (although discriminately. for economic rea-
`sons) to achieve optimal therapeutic results. Simpler treat-
`ment delivery techniques that yield an acceptable dose
`distribution should be preferred over more costly and
`complex ones.
`in which a greater margin of error on a
`day-to-day treatment basis may be present. Repositioning
`and immobilization are critical because the only effective
`
`irradiation is that which strikes the clonogenic tumor
`cells. Therefore, in fractionated irradiation accurate setup
`should be such that the patient will maintain the desired
`position during every daily treatment. Repositioning and
`immobilization devices. such as the Alpha cradle, plaster
`casts,
`thertnoplast molds. bite blocks, and arm boards.
`are
`invaluable in assisting technologists
`in patient
`positioning.
`Accuracy is periodically assessetl with portal (localiza-
`tion) films or on—line imaging verification (electronic por-
`tal
`imaging) dcv'ices.""’“"""""" Portal
`localization errors
`may be systematic or occur at random. On-line electronic
`portal imaging has been used to document inter- or intra-
`trcatment portal displacement in patients treated with pel-
`vic irradiation.“ lntertreatment displacement exceeding
`10 mm was seen in 3"’~ in the tncdiolateral, l6% in the
`craniocaudal, and 23% in the anteroposterior direction.
`There was no intratreattnent displacement exceeding 10
`mm in 547 images.
`In a review of 48 patients on whom multiple digital
`portal verification images were obtained, Bissett and col-
`leagues“ noted that displacements of the field were 2.9
`tntn in the transverse and 3.4 mm in the craniocaudal
`dimensions. Mean rotational displacement was 2 degrees.
`The mean treattnent field coverage in this set of images
`was 95%. There were some variations in the assessment
`of the translational errors when observations of several
`radiation oncologists were analyzed.
`Rabinowiu. and colleagLtes.'m in a comparison of simu-
`lator and portal lilms of 71 patients, noted some discrep-
`ancies between the simulator and the localization (treat-
`ment) portal
`films. With an average value of 3-mm
`standard deviation of the variations, the mean worst case
`discrepancy averaged 3.5 mm in the head and neck region.
`9.2 mm in the thorax, 5.1 mm in the abdomen, 8.4 mm
`in the pelvis. and 6.9 mm in the extremities. Other investi-
`gators have documented similar localization errors on the
`basis of portal lilm review analysis.3"""‘”7
`
`Page 6 of 27
`
`

`
`TABLE 1 -1. Carcinoma of nasopharynx: correlation of quality of pontal Iilms with primary tumor control
`Local tumor control
`
`(J11.-\P'1'1«:R l:
`
`()Vl“.R\-"ll;‘\-\’
`
`/ 5
`
`Simulation done
`Simulation not done
`
`>-75% adequate portal films
`<:75% adequate portal films
`
`1956-1965
`(n = 30)
`0
`18/30 (60%)
`8/11 (73%)
`10/19 (53%)
`
`1966-1975
`(n = 54)
`12/21 (57%)
`16/33 (52%)
`16/28 (57%)
`12/24 (50%)
`
`1976-1986
`(n : 59)
`43/57 (75%)
`0
`42/56 (75%)
`(100%)
`1/1
`
`Percent of films with ear block shielding nasopharynx
`—;25°/o
`26-50%
`>51 °/o
`
`11/17 (65%)
`7/12 (58%)
`
`0/1
`
`22/38 (61%)
`4/9 (44%)
`2/7 (29%)
`
`42/56 (75%)
`1/1 (100%)
`0/0
`
`Total
`(n ~ 1413)
`55/78 (71%)
`34/61 (56%)
`P : 0.1
`66/96 (69%)
`23/44 (52%)
`P=om
`
`75/109 (69%)
`12/22 (55%)
`2/8 (25%)
`P - 0.04
`
` _
`(Perez CA, Devineni VR, Marcial-Vega V. et al: Carcinoma of the nasopharynx: Factors affecting prognosis. Int J
`Radiat Oncol Biol Phys 232271-280, 1992)
`
`Hettdricksonm“ reported it 3.5% error frequency in mul-
`tiple parameters (setting of lield size. timcr. gantry and
`collimator angles. and patient positioning) with one tech-
`nologist working. The error
`rate declined to 0.82%
`when two technologists worked together. Marks and co-
`w'orkcrs2““”5 demonstrated, by systematic use of verifi-
`cation films, a high frequency of localization errors in
`patients irradiated for head and neck cancer or malignant
`lymphomas. These errors were correctetl with improved
`patient immobilization; with the use of a bite block in
`patients with head and neck tumors locali7.ation errors
`were reduced from 16% to l%.”"‘
`Doss,‘"’ in a study of patients with upper airway carci-
`noma, showed that in 21 of 28 patients (75%) with treat-
`ments in which 30% or more portals exhibited a blocking
`error. a reeunence developed. whereas tumor failure was
`noted in only 2 of I2 patients (17%) without such cn'ors.
`Perez and colleagues'”“ also reported a higher incidence
`of failures in patients with carcinoma of the nasopharynx
`on whom shielding of the ear inadvertently caused some
`blocking of tumor volume (Table 1- l).
`Suit and associates” reviewed various recent techno-
`logic developments that through more precise trcattnent
`planning and delivery techniques will reduce volume irra-
`diated and improve dose distributions, which should en-
`hance therapeutic outcome.
`
`RELEVANCE OF RADIOBIOLOGIC
`CONCEPTS IN CLINICAL RADIATION
`THERAPY
`
`Clinical radiation therapy has evolved primarily from
`empiricism. Nevcrtliclcss,
`in the past 30 years a major
`effort has been applied to the potential application of
`radiohiologic concepts to design safer and more effective
`therttpeutic strategies. Kaplanm pointed out that. although
`direct extrapolation from in vitro and in vivo experimental
`data may not have resulted in spectacular advances in
`
`these biologic concepts have
`clinical radiation therapy.
`greatly cnltuncetl our understanding of the principles stir-
`rounding the clinical use of ionizing radiation. Experi-
`ments on t'atli:ttion tlamage to DNA (both single- and
`double-strand scissions) and its repair have facilitated un-
`derstanding of repair of sublcthal and potentially lethal
`dztrnttgem W and provided the rationale for manipulation
`of dosc—time relationships.”°
`One of the most significant contributions of radiation
`biology has been the theory of cell kill as a function
`of increasing doses of a cytotoxic agent. as well as the
`demonstration of repair of sublethal or potentially lethal
`dtunage after irradiation.'3“""‘3‘5"' These concepts have led
`to a better understanding of dose-response curves for tu-
`mor control probability and effect on normal tissues and
`applic.-.ition of dose-time concepts to fractionation. Studies
`on the role of oxygen and the demonstration of hypoxic
`cells in tumors and their impact on sensitivity to irradia-
`tion”" was another important contribution leading to the
`concept of reoxygenationm and the potential use of hy-
`pcrbaric oxygen or hypoxic radiation settsitizcrs in clini-
`cal radiation thet'apy.l""‘°'6
`Hiickcl and associtttcsz” pointed out that tumor oxy-
`genation measured with the polarographic needle elec-
`trode method was a powerful predictor of radiation ther-
`apy outcomc in patients with locally advanced carcinoma
`of the uterine cervix. The 5-year survival rate was about
`75% for 21 patients with a median p0) greater than l()
`mm Hg versus 40% for 23 patients with a median p02
`less than 10 mm Hg.
`the biologic
`The study of cell proliferation kinetics,
`basis of cell killing by irradiation or chemotherapeutic
`agents, and the effectiveness of each modality in specific
`cellular compartments has strcngtltencd understanding of
`combination tl1crapy.""""‘””"""" The same can he said for
`the use of various combinations of irradiation and surgery
`to decrease locoregional recurrences or to exploit the spe-
`cific ability of each modality to eradicate tumor cells in
`
`
`
`Page 7 of 27
`
`

`
`n_l R:t.'lu.'r"v-at (Hi: -»iu;;\
`l’r1/tti,'>lc.v wit! l’It:t‘Iu:
`lliatly
`I.t't.H'i_Il'.i1Iirni.
`:‘tlI
`l by (V /\
`l’crc'I and l
`\k'
`l.ipptt'.cotl Raw
`l’:ihltsl.eix. l".'iil.tdc|plii.t
`IWU
`
`
`
`t
`
`CIIAPTER 8
`
`Principles ()f Radiologic Physics, Dosimetry,
`and Treatment Planning
`
`.]2llllCS A. Purely
`
`
`A solid loundation in the principles of radiologic physics.
`dosimetry. and treatment planning is essential
`for the
`practice of radiation oncology.
`In this chapter, we con-
`sider several
`topics that
`lay the basis for the material
`covered in Chapter 9. This chapter also discusses basic
`concepts used in calculating the (lose administered to a
`patient and the standard correction methods used to ac-
`count for air gaps and tissue inhomogeneities.
`
`ATOMIC AND NLICLEAR STRUCTURE
`
`The atom ntay be thought of as consisting of a centrally
`located core, the nucleus. surrounded by small orbiting
`particles, electrons. The overall dimension of the atom is
`about
`It) "' Ill, the nucleus about 10 '4 tn.
`.\/lost of the
`mass of the atom is contained in the nucleus. making it
`extremely dense ([0 ‘ kg/ml). The nucleus is composed
`ol‘ two kinds oi’ panicles. protons and neutrons. known
`collectively as nucleons. A proton has a mass (mp) of
`1.673 X 10 2' kg and has a positive electrical charge
`equal
`in magnitude to the charge of
`the electron
`(1.602 —- l0
`cottlomb_). Collectively. the protons consti-
`tute the electrical charge of the nucleus. A neutron is
`slightly more massive than a proton (111,, = 1.675 X 10 27
`kg) and has no electrical charge. Each negatively charged
`electron has a rest mass (rim) of 9.l |() X 10 ll kg, contrib-
`uting little mass to the nucleus.
`In 1913, Niels Bohr formulated a planetary model of
`the hydrogen atom. consisting of an electron orbiting
`around a nucleus of equal and opposite charge.
`In ex-
`tending his theory to multielectron atoms. Bohr proposed
`a nucleus surrounded by electrons arranged in concentric
`shells or energy levels (Fig. 8-1). Energy is released
`when an electron moves to an orbit closer to the nucleus.
`
`outward. by the letters K. L, N], and so forth. There is a
`maximum number of electrons that can be accommodated
`in each shell:
`two in the first shell. eight in the second.
`eighteen in the third. and so on.
`The maximum number of electrons allowed in each
`
`is given by the relat.ionship 2n’; n is an integer
`shell
`specific to each shell and is called the principal quantum
`number. Other properties of the electron also have discrete
`values specitied by quantum numbers. These include the
`e|ectron’s angular momentum as it orbits the nucleus.
`denoted by quantum numberl (l : 0. 1.
`.
`.
`.
`. n
`1):
`its spin about its axis. denoted by s (s : ‘.1/2); and its
`magnetic moment, denoted by m. (m. = t), +1,
`.
`.
`.
`.
`:1). Thus. each electron in an atom has an associated set
`of quantum numbers (n,
`l. s, m,). This is the basis ol
`the Pauli exclusion principle, which states that no two
`electrons can have the same set of quantum numbers
`within a particular atom.
`.\r1odern physics has replaced the simplistic orbiting
`electron model of Bohr with an abstract model of diffuse
`
`electron clouds that represent probability functions of the
`electrons position. However, for understanding of radio-
`logic physics. the simple model of a nucleus composed
`of protons and neutrons and surrounded by clcctrotts‘ is
`sufficient.
`
`The atom ol‘ an element is specified by its atomic num-
`ber. denoted by the symbol Z, and its mass number, de-
`noted by the symbol A. The atomic number is equal to the
`number of protons in the nucleus. and the mass number is
`equal to the number of nucleons (protons and neutrons)
`in the nucleus. Hence. A minus Z is equal to the number
`of neutrons. denoted by the symbol N. within the nucleus.
`In addition. each element has an associated chemical sym-
`bol. When these definitions are used, the standard notation
`
`and energy is required to move an electron into a higher
`orbit. I-listorically, the shells are labeled, from innermost
`
`to specify an atom is ;‘_X as illustrated by 3‘-’Co, which is
`a radioactive isotope of the element cobalt that has an
`
`243
`
`
`
`Page 8 of 27
`
`

`
`(Ill.A\P'Il-‘.R 8: Pl{l.\(1ll’l.l.S‘ or R.-\I)l()l_,()(.‘.l('.
`
`l’ll\T\'|(‘S.
`
`l)osi.\ii-“TRY.
`
`.«\Nn 'llRl".»\'l't\tll".\"l‘
`
`l’I,.-‘\\',\'l.\l(‘.
`
`2:')l
`
`QC
`
`100
`
`80
`
`60
`
`A0
`
`PHOTOELECTRIC EFFECT
`DOMINANT
`
`PAIR PRODUCTION
`DOMINANT
`
`COMPTON EFFECT
`DOMINANT
`
`20 ATOMIC
`NUMBEROFABSORBER
`
`out
`
`o t
`
`1
`PHOTON ENERGY (Mev)
`
`10
`
`100
`
`FIGURE 8-16. Relative importance of the three principal
`modes of interaction as a function of photon energy and
`atomic number of medium. (Hendee WR: Medical Radiation
`Physics, ed 2. Chicago. IL. Year Book Medical, 1979)
`
`ln photodisintcgration, a high-energy photon interacts
`with the nucleus oi’ an atom, totally disrupting the nucleus.
`with the emission of one or more nucleons (Fig. 8-15).
`It typically occurs at photon energies much higher than
`those encountered in radiation therapy.
`
`RADlA'l'l()N TH ERA |’Y '|' l{EA'l‘MEN'l‘
`MACHINES
`
`Kilovoltage Units
`
`Before 1951. most radiation treatment units were x-ray
`machines capable of producing photon beams having only
`limited pcnctrability. In these machines. the electrons are
`accelerated by an electric field produced from a high Volt-
`age generated in a transformer that
`is applied directly
`between the lilaineut
`(cathode) and the x—ray target
`(anode). A schem;ttic diagram of a radiation therapy
`x—ray tube is shown in Fig. 8» I 7. The potential difference
`fkVp) is variable on these machines. and metal filters can
`be added to absorb the lower—encrgy photons preferen-
`tially, changing thc penetrability of the beam. The combi-
`nation of Variable k.Vp and different filtration provides
`the capability of generating multiple x—ray beams. The
`degree of penetrability is used to categori7c the units as
`
`an energetic photon approaches closely enough to the
`nucleus of the target atom.
`the incident photon energy
`may be converted directly into