Bulski et al.: Optimization of Dose ...

OPTIMIZATION OF DOSE DISTRIBUTIONS IN INTRAOPERATIVEHDR BRACHYTHERAPY

Bulski W1, Kawczynska M1

, t.yczek J 2, Olszewska A1

1Medical Physics Dept. and 2Srachytherapy Dept., M. Sktodowska-Curie Memorial Centre of Oncology,Roentgen str.5, 02-781 Warsaw.

Received 17 December 1998; revision received 8 February 1999; accepted 23 November 1999.

Key words: HDR, brachytherapy, intraoperative radiotherapy.

ABSTRACT

The authors present their initial experience in phy.sical aspects of treatment planning in HDR intraoperativebrachytherapy. The examples are given of implantations in various tumour localizations: head and neck, pancreas,soft tissue sarcomas in the abdomen. The technical and dosimetric problems which may occur in such situations arediscussed. The capabilities of dose distribution optimization by the Abacus HDR treatment planning system arepresented.

INTRODUCTION

Intraoperative radiotherapy (IORT) is apromising modality with which it is possible toincrease tumour dose and, therefore, tumourcontrol, without increasing therapy-inducedmorbidity. It has the distinct advantage ofdirectly irradiating the tumour bed, whileeliminating the surrounding normal tissues andorgans from the field of treatment. Thecombination of maximal tumour irradiation andminimal normal tissue irradiation can maximizethe therapeutic ratio and the efficacy of thetreatment. Intraoperative radiotherapy isespecially useful when the required radiation.dose exceeds the tolerance dose of thesurrounding normal tissues.

However, the application of IORT withexternal beams has been significantly limitedby cost, logistic issues, and technicalproblems. On the other hand, intraoperativebrachytherapy makes it possible to delivertreatment to difficult anatomic areas. The highdose rate (HDR) afterloading brachytherapy isa very suitable method, being cost effective,logistically sound, and suitable for a widerange of anatomic sites (Verniers, 1992). Theinherent possibilities of remote afterloadingHDR techniques offer several importantadvantages over conventional LDRtechniques. They improve radiation protectionof the medical and nursing staff, andsignificantly facilitate the optimization of dosedistribution.

Rep. Pract. Oneol. Radiother. 4 (1) 1999

The intraoperative HDR brachytherapy inmost cases is performed as a multi-fractionirradiation when the HDR catheters are placedand sutured to the tumour bed during thesurgery and left there with their open endssticking out through the surgical scar. Severaldays after the surgery, when the post surgeryswelling has stabilized, the actual catheterposition may be accurately established byradiography and irradiation may be planned.

The intraoperative brachytherapyapplications in our department are limited sofar to pancreatic tumours, soft tissuesarcomas, and head and neck tumours. Theaim of this paper is to present our initialexperience in physical aspects of treatmentplanning and dose distribution optimization inHDR intraoperative brachytherapy.

MATERIALS AND METHODS

Since 1996 we have treated intraoperativelyover 50 patients for pancreatic tumours, softtissue sarcomas, and head and neck tumours.All the irradiations were performed using theGammamed 12i stepping source afterloaderwith the Ir-192 source provided by theIsotopen-Technik Dr.Sauerwein GmbH. Theactivity of the source was between 10 Ci (newsource) and 5 Ci (before source replacement).The treatment times ranged between 5 - 15minutes per fraction.

Dose distributions were calculated andoptimized using the Abacus 1.6 treatment

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Bulski et al.: Optimization of Dose ..,

planning software from the same company.3-D dose volume histograms (DVH) were usedfor comparison of various treatment plans.A detailed analysis of the functioning of theoptimization algorithm will be the topic ofanother paper.

PancreasIntraoperative interstitial brachytherapy has

been used during laparotomy for tumourslocalized in the pancreas. The volume to beimplanted is determined by measuring threemutually perpendicular dimensions of thetumour during laparatomy [Bodner, 1997]. Theextent of the tumour is marked by surgicalclips. Flexible guide tubes are used. The sizeand shape of the tumour determine thenumber of catheters. The catheters areinserted into the pancreatic tumour and are asparallel to each other as possible. The optimaldistance between the catheters is approx. 1em but in clinical situations in most cases it isimpossible to fix them parallel to each other.The catheter localization was done by twoorthogonal X-ray films and CT examination.The target volume was defined as a volumeincluding all surgical clips with a 1 cm margin.In a group of 30 patients treated this way, thetumour dose was 30 Gy in daily fractions of 3Gy in a continuous regime (including week­end). Seven days later, brachytherapy wasfollowed by a radio-chemotherapy treatment ,with the external beam tumour dose of 45 Gy,combined with two 5 day courses of 5Fu as aradiosensitizer. In most cases, a three fieldtechnique was used: one frontal field and twolateral wedge fields.

Head and NeckWhen cancer recurs in an area of earlier

radiation, further external beam irradiation isusually impossible. Salvage surgery alone hasa low curative potential and, therefore, shouldbe combined with interstitial intraoperativebrachytherapy (Harrison, 1997). Because ofthe specific anatomical situation, preciseintraoperative tumour volume definition is verydifficult. In these regions, large margins (2.5em) around the resected area should beirradiated. Flexible guide tubes are implantedparallel to each other with a 1.5 cm distance,between them. The day after the operation thelocalization of the catheters is established bytwo orthogonal X-ray films. Target volumedelineation is based on the pre-surgery CTexamination, check films and intraoperativeexamination. In a group of 12 patients treatedup till now, a tumour dose of 40 Gy wasdelivered in 3 Gy fractions two times a daywith a 6 hour interval (13 fractions in 7 days).

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Soft tissue sarcomas (STS)In STS two different clinical situations occur.

When the tumour is localized in theextremities, the target volume delineation isrelatively easy. The tumour bed after primarytumour resection should be encompassed bythe isodose corresponding to the curativedose. The number of catheters is determinedby the size of the tumour bed and in mostcases it is possible to place them fairly parallelto each other and in equidistant positions(Harrison, 1993). When the STS is localized inthe extraperitoneal region it requires a cyto­reductive operation and careful placement ofsurgical clips. Target delineation is basedmainly on the orthogonal X-ray films, CT scansand intraoperative examination. The marginsshould be as large as possible, a limiting factorbeing the neighbouring late reacting organs atrisk [Hilaris, 1997]. Dosage and fractionation inour group of 9 patients were individualizedaccording to the clinical and histologicalsituation and to previous radiotherapytreatment.

DISCUSSION

Once the catheters are placed in the patient,stepping source brachytherapy offers twodegrees of freedom: the dwell position and thedwell time. Usually the dwell positions areplaced in the sections of the catheters, whichare inside the target volume. Then optimizationof the dose distribution is performed bymanipulation of the dwell times by dedicatedsoftware. Because brachytherapy radiationbegins within two-three days after theoperation, it gives the physicist enough time towork out an optimal treatment plan. Theoptimized isodose distribution should matchthe requirements specified by the physicianand fulfil the recommendations of the ICRU ondose and volume specifications for reportinginterstitial therapy (ICRU, 1997).

Despite the efforts of radiotherapists to followthe rules of proper catheter placement duringthe implantation procedure, in numerous casesthe resulting distribution of catheters is far frombeing ideal. According to our experience, theHDR treatment planning may pose certaintechnical problems. The eventual position ofthe catheters may differ from that planned,some of the catheters may get bent to such anextent that the source cannot be entered intothem, etc. In Figs. 1 and 2 we show twopancreatic tumour implants. One implantfollows relatively well the rules of the ParisSystem, while the other clearly ,does not. Fig. 3presents the dose distribution in 3D for thelatter implant, which was attained thanks to the

Rep. Pract. Oncol, Radiother. 4(1) 1999

Bulskl et al.: OptimIzation of Dose ...

optimization procedures of the Abacus treatmentplanning system. The optimization is based on aset of predefined dose points at the borders ofthe target volume which are supposed to receivea predefined dose. The dose points areautomatically generated by the planning softwareat equal distances from peripheral catheters.

The mathematical optimization, calledoptimization on distance, aims at determining

the dwell positions and relative dwell timessuch that the reference isodose surfacepasses through these dose points (Van derLaarse, 1991). The 1.6 version of the Abacusplanning software provides the option ofentering additional user-defined dose pointsinside the target volume, which improvesoptimization capabilities.

Fig. 1. Pancreatic tumour implant The catheters are fairly parallel to each other. It is relativelyeasy to get afomogeneous dose distribution.

FIg. 2. Pancreatic tumour implant. Six catheters within thetarget volume are clearly not parallel to each other,which largely complicates the optimization of thedose distribution.

Rep. Pract. Oneal. Radlother. 4 (1) 1999

Fig. 3. The dose distribution in 3-D for the implant presentedin Fig. 2. The optimization procedures of the planningsoftware made it possible to get quite satisfying dosedistribution for a relatively irregular implant. The 3 Gyisodose surface is presented together with lheposition ofcatheters (red) and reference points (blue).

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Bulskl et al.: Optimization of Dose ...

The next set of illustrations presents anexample of the brachytherapy of the STS inthe abdominal region. Five guide tubes wereimplanted fairly parallel to each other.However, it turned out that one of them wastoo tightly sutured and the source could notenter into it. The treatment plan had to beprepared for four out of five catheters. Theoptimization was still based on the dose pointsas defined for the five catheter implant.

Fig. 4. Soft tissue sarcoma (STS) implant in the abdominalregion. Five catheters are implanted fairly parallel toeach other. One of the catheters, too tightly sutured,did not let the source in. Later, during the treatment.the second catheter became unusable and thetreatment had to be re-planned. The position of activeand inactive catheters is shown in Figs. 5 and 6.

The resultant dose distribution is presented inFig. 5. After several fractions the situationbecame even worse because one morecatheter became unusable. The treatment hadto be re-planned and the resultant dosedistribution is presented In Fig. 6. Thereference isodose still encompasses the targetvolume. The analysis of the 3-D DVH's for thetwo plans shows that the volumeencompassed by the 1.5 Gy isodose

- (reference Isodose optimized to the same setof predefined reference points) increased onlyby 5% and that of 1 Gy isodose by 7.5%. Thevolume of 2.25 Gy isodose (150% of thereference dose) decreased by about 1% andthe volume of 3 Gy isodose decreased by 7%.This proves the versatility of the Abacusoptimization procedure. The relatively smallchanges in dose homogeneity are not likely tocause any late complications.

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Fig. 5. The dose distribution for the implant presented in Fig.4. Four out five catheters (1, 2, 3, 4) are used fortreatment. The reference isodose of 1.5 Gy coversthe target volume.

Fig. 6. The dose distribution for the implant presented in Fig.4. Three out of five catheters (1, 3, 4) are used fortreatment. The reference isodose of 1.5 Gy stillencompasses the target volume. The difference inhomogeneity is discussed in the text.

Rep. Prac!. Oneal. Radio!her. 4 (1) 1999

Bulski et al.: Optimization of Dose ...

To summarize, it should be emphasized thatthe quality of brachytherapy applicationsdepends on the choice of the target volume,dose distribution homogeneity, and onradiation injury to critical tissues. The individualadjustment of the dose shape to the targetvolume by careful treatment planning isessential for better treatment quality. Imagingmethods play a significant role in planningprocedures. In order to define the targetvolume properly various imaging techniquesmay be used: orthogonal X-rays, CT, etc. Thedose distribution calculations should beperformed in 3-D for better evaluation of thetreatment plan. However, one should bear inmind that with all the computer power, displaytechniques and optimization methodsavailable, the degree to which a brachytherapyimplant will be effective is determined not byhow well the implant is optimized, but how wellthe physician has physically placed thecatheters or applicators. The optimizationsoftware cannot provide a good dosedistribution around a badly placed implant.

CO'NCLUSIONS

1. If the placement rules for the catheters of agiven target volume are not adhered to, it isdifficult, if not impossible, to obtain a gooddose distribution;

2. A mathematically optimized dose distributiondoes not always represent the best possibledistribution in and around the implant, itdepends very much on the number andpositions of the selected dose points;

3. If an implant is not covering the targetvolume geometrically, special techniques,manual or' mathematical, of changingrelative dwell times may be required tocover the target volume with the referenceisodose surface;

4. Clinical experience is always required toevaluate the mathematically optimized dosedistribution for actual patient treatment.

Rep. Praet. Oneol. Radiother. 4 (1) 1999

REFERENCES

Bodner W R, Hilaris S B: Brachytherapy andPancreatic Cancer. Seminars in Surgical Oncology(1997); 13: 204-207.

Harrison L B: Applications of Brachytherapy in Headand Neck Cancer. Seminars in Surgical Oncology(1997); 13: 177-184.

Harrison L B, Zelefsky M J, Armstrong J G,Schlupak K 0, Brennan M F: Brachytherapy andfunction preservation in localized management ofsoft tissue sarcomas of the extremity. Seminars inRadiation Oncology (1993); 3: 260-269.

Hilaris S B, Bodner W R: Role of Brachytherapy inAdult Soft Tissue Sarcomas. Seminars in SurgicalOncology, (1997); 13: 196-202.

ICRU Report 58: Dose and Volume SpecificationFor Reporting Interstitial Brachytherapy. ICRU(1997).

Van der Laarse R, Edmundson G K, Luthmann RW, Prins T P E: Optimization of HDR brachytherapydose distributions. Activity Journal (1991) 5: 94-101.

Verniers 0 A A, Koper P C M, Levendag P C:Intraoperative radiation: review of clinical data and futureperspectives for advancecl/unresectable tumours.Selectron Brachytherapy Journal (1992); 6: 100-105.

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